Monolithic microwave integrated circuits (MMIC) have well known uses in electrical circuitry operating in the microwave/millimeter-wave range. These electronic components are used in communication and navigation systems, tactical and strategic sensors, and electronic warfare. A preferred material in making the MMIC chips is gallium arsenide (GaAs). Because complex GaAs chips may have certain fabrication and operation problems, it is usually more convenient to make high level circuits or "modules" from a plurality of MMIC chips thus creating an integrated circuit assembly. A common module is a microwave/millimeter-wave transmit/receive module for use in phased array systems employed in satellite communications and radar systems.
Each MMIC chip may contain several microwave or millimeterwave circuits such as amplifier, mixer, or oscillator circuits depending on the function and level of integration. For interfacing with both DC and RF signals and currents, a MMIC typically has contact pads around the perimeter of its top surface. MMIC chips may be mounted in recesses or gaps in a carrier surface such as a substrate, and interconnections between contact pads on the chip surface and interconnect metallizations on the chip carrier are made of bonding wires or ribbon. In the case of RF connections, either microstrip or coplanar waveguides are used for waveguide transmission over the carrier. Carriers which use microstrip lines typically have a conducting ground plane on the bottom surface of the carrier in opposing relation to the top conductor lines. Impedance of the strip line is primarily controlled by line geometry, such as width and thickness, or proximity to a ground plane.
Advanced phased array applications generally dictate a very large number of antenna elements in the array to support high gain or large directivity requirements. In a typical application for extremely high frequency (EHF) 30-300 GHz antennas, a given array consists of 3000-5000 elements interspersed in a rectangular array. In an active aperture, array elements are associated with each of the antenna elements. Because of the large number of antenna elements required, some sort of high density interconnection of the MMIC chips is required.
Efficient and low cost interconnection of such MMIC chips becomes a major challenge for successful module performance, especially in high frequency, large array applications. Within the modules, which tend to become quite small at higher frequencies, the individual chips should be interconnected by connections which preserve transmission line quality (i.e., maintain transmission line impedances and avoid discontinuities causing reflections) and which are short to minimize unnecessary time delays in processing the signals.
Existing modules attempt to offer economical interconnection methods between MMIC's in the module while providing a production oriented design. One such module uses a grid array resembling a waffle. This package contains a series of dielectric coated wires running between recessed receiving locations or unpackaged chips. Layers of metal foil between wires minimize cross coupling. This system also requires manufacturing intensive wire routing to the appropriate devices. However, coupling to a waveguide must be done through an additional transition piece (probe pins). Although the subsystems may provide acceptable interconnection between multiple MMIC's, they require the employment of a separate waveguide coupler and are relatively high in cost due partially to their fabrication complexity.
Two other types of modules either employ a coplanar method or a coaxial feedthrough. In the coplanar method, a large substrate has multiple recesses formed into its surface which are each sized to receive a MMIC chip. Conductive striplines are patterned over the top surface of the substrate to provide interconnection between the MMIC chips. The drawback of this type of module, is that each of the MMIC chips must be properly spaced so as not to interfere with the operation of another chip. Positioning all the MMIC chips in one plane takes up a significant amount of area which goes against some of the high density requirements of the phased array antenna.
In a coaxial module, the MMIC's are positioned in substrate chip carriers. The chip carriers are then stacked one on top of the other. A feedthrough is then created through each of the chip carriers to provide interconnection between each of the MMIC's at the different levels. The size of the feedthrough with respect to the required impedance can be large, the fabrication process is difficult, and the interfacing of the circuits on different levels creates an additional packaging problem.
Therefore, what is needed is a MMIC module which does not take up a great amount of area and is easy to manufacture. It is important that a large number of MMIC's are located in a small area without the problems of interference and poor electrical interconnection between the integrated circuits.